When a transmission line develops a fault by shorting or grounding, a large current flows in the transmission line and the power distribution system is shut off in an electric power substation or the like, so that the power supply to the transmission line is interrupted. In such a case, it is necessary to immediately locate the faulty point to cope with the fault.
In this regard, various types of devices and systems have been devised and put into practice in order to locate such faulty points or sections of transmission lines.
In a technique for locating such a faulty point, a current detector is provided in each of a plurality of spots of a transmission line, to detect a difference between the arrival times of detected signals received from two spots, the location of which is known.
The principle of such a time difference detection method is briefly described with reference to FIG. 1A.
Referring to FIG. 1A, current detectors (CT) 2a to 2e are provided in a plurality of spots A, B, C, D and E of a transmission line 1, in order to detect the current flowing in the transmission line 1 at these spots. Further, time difference detection circuits 3a to 3d are arranged for detecting a difference between arrival times of signals received from a pair of neighboring current detectors. The time difference detection circuit 3a detects the difference between the arrival times of signals received from the current detectors 2a and 2b, while the time difference detection circuit 3b detects the difference between the arrival times of signals received from the current detectors 2b and 2c. The time difference detection circuit 3c detects the difference between the arrival times of signals received from the current detectors 2c and 2d, while the time difference detection circuit 3d detects the difference between the arrival times of signals received from the current detectors 2d and 2e. Each of these time difference detection circuits 3a to 3d includes a counter, for example, which is started to count when a fault current detection signal exceeding a prescribed level is received from one of the pair of current detectors, and stops its counting when another fault current detection signal exceeding the prescribed level is received from the other current detector.
For convenience of illustration, it is assumed that the transmission line 1 is divided into sections at regular intervals l by the spots A to E in the structure shown in FIG. 1A. It is further assumed that the time difference detection circuits 3a to 3d are provided at the central points of the respective sections. The operation of this structure will now be described. It is assumed that a spot P between the spots B and C develops a fault. At this time, a large surge current flows toward the spots B and C due to the fault. It is assumed that the length of a section BP is l1, the length of a section PC is l2 and the surge current flows at a velocity V.sub.S, while each current detector transmits a signal to each time difference detection circuit through an optical fiber cable at a velocity V.sub.0.
At this time, the leftward flow of the surge current requires the following time Tb to arrive at the spot B: EQU Tb=l1/V.sub.S
Similarly, the rightward flow of the surge current requires the following time Tc to arrive at the spot C: EQU Tc=l2/V.sub.S
Similarly, the surge or fault current requires the following times Ta, Td and Te to arrive at the spots A, D and E: EQU Ta=(l/V.sub.S)+Tb EQU Td=(l/V.sub.S)+Tc EQU Te=(2l/V.sub.S)+Tc
Since the distances between the respective current detectors and the respective time difference detection circuits are equal to each other and the signals are transmitted through the optical fiber cables at the same velocity, time differences detected by the time difference detection circuits 3a to 3d are expressed as follows: EQU Da=Ta-Tb=l/V.sub.S EQU Db=Tb-Tc=(l1-l2)V.sub.S EQU Dc=Tc-Tb=(l/V.sub.S) EQU Dd=Td-Te=l/V.sub.S
where Da to Dd represent the time differences detected by the time difference detection circuits 3a to 3d respectively.
As understood from the above equations, the time difference Db detected by the time difference detection circuit 3b differs substantially from those detected by the remaining time difference detection circuits 3a, 3c and 3d. Thus, it is possible to detect that the faulty section is between the spots B and C by observing the distribution of these time differences, while the faulty point itself can be located by observing the values of the time differences.
FIG. 1B is another block diagram showing the structure of the system using a time difference detection method. Each discriminator 12 includes four counters 201 to 204. The first counter 201 receives a surge signal from a current transformer CT1 to start counting and stops the counting by a stop signal from the second counter 202, which has received a surge signal from a current transformer CT2. Similarly, the second counter 202 receives the surge signal from the current transformer CT2 to start counting and stops the counting by a stop signal from the third counter 203, which has received a surge signal from a current transformer CT3 through a combining filter 205 and a branching filter 206.
When the surge signals from the current transformers are increased, it is possible to detect surges among a large number of spots by increasing the number of the counters.
The time differences detected by the counters 201 to 204 or the count values thereof are transmitted to the central unit 22 by a data transmission unit 207, and stored in a disk unit 213 through an O-E converter 21 and a personal computer 212. The personal computer 212 computes a surging spot, which in turn is displayed on a CRT 214 and printed out by a printer 215.
While each discriminator handles four current transformers in this example, it is possible to increase the number of such current transformers handled by each discriminator through a combining or branching filter. Alternatively, each discriminator may handle a minimum unit which includes two current transformers.
As another method of similarly using a surge current, it may be considered to detect a transmission line section where the surge current changes direction through surge phases which indicate that this section has developed a fault, thereby to locate the faulty section.
FIG. 3 is a perspective view showing a conventional faulty section locating system using the current resulting from a fault.
In the system shown in FIG. 3, a plurality of detectors are distributed along the transmission line. Data obtained by the detectors are collected for every prescribed line section by transmission means, such as an optical fiber cable or the like. The collected data are compared with each other for making decisions, and the results of comparison and/or decision are transmitted to a central unit or the like, for a synthetic evaluation.
Referring to FIG. 3, a transmission line 1 is strung along steel towers 10, which are installed at appropriate intervals. An optical fiber composite overhead ground wire (OPGW) 13 is mounted on upper parts of the steel towers 10 along the transmission line 1. Detectors 11 such as current transformers (CT) are mounted on the optical fiber composite overhead ground wire 13 at prescribed intervals. Further, discriminators 12 are mounted on prescribed ones of the steel towers 10 in order to receive detection signals from the detectors 11. Optical fiber cables 14 are connected between the detectors 11 and the discriminators 12. The discriminators 12 are connected to a central unit 22 by an optical fiber cable 17 provided in the optical fiber composite overhead ground wire 13. The discriminators 12 also serve as terminals for the data transmission.
The above mentioned conventional method is adapted to detect phases of a dynamic current flowing when a fault occurs thereby to locate a faulty section through changes of such phases and/or current levels.
However, the aforementioned conventional methods have the following problems:
(1) In the time difference detection method shown in FIG. 1, it is decided that a fault has occurred in a section having a time difference between signal arrival times which is different from those in other sections. Therefore, if a current detector becomes inoperable due to any cause, no time difference can be detected in a seciton adjacent to the inoperable current detector. Thus, it may be impossible to correctly locate a faulty section.
(2) When phase changes, levels, etc. of surge currents are detected to locate a faulty section, it is necessary to detect different phases or amplitudes of the surge current at the same time instant. In this case, however, it may be impossible to detect the current at the same time instant due to differences in response times caused by dispersion of the characteristics of the current detectors and due to differences of delay times following a signal propagation.
If the phases of a surge current flowing in a plurality of spots of a transmission line can be detected at the same time instant as shown in FIG. 2A, the distribution of the current phases can be correctly detected along the transmission line, thereby to locate the faulty section. However, when the surge current phases are detected at different time instants in the respective detection spots as shown in FIG. 2B, it is impossible to correctly detect the phases. Consequently, the phase distribution of a fault current thus obtained differs from the true phase distribution, whereby a correct detection of the fault current is impaired.
(3) In the method of detecting a dynamic current, the faulty section and spot are located only if a fault taking place in the transmission line, generates a dynamic current in response to such a fault. Otherwise, it is generally decided that a noise has occurred requiring a resetting of the respective detectors from a central monitoring station.
In a general overhead transmission line, however, it is not unusual that no fault current flows in the transmission line even if an overhead ground wire provided for protecting the overhead transmission line against a lightning stroke, has in fact been struck. In a recent monitoring system for an overhead transmission line, on the other hand, monitoring data from monitoring spots for the transmission line are transmitted through an optical fiber cable which is combined with the overhead ground wire.
When such an optical fiber composite overhead ground wire is struck by lightning, the optical fiber cable serving as a communication line, may be damaged which hinders the transmission of the monitoring data. Consequently, it may be impossible to correctly transmit the monitoring data of the transmission line to a central monitoring station, for example.
The conventional monitoring system has no structure for detecting only such a phenomenon when a line and its data transmission line have been struck by lightning. When a communication failure is caused by lightning, therefore, maintenance engineers of the power company must patrol along the entire transmission line to detect a spot struck by lightning. Thus, much labor and man-hours are required to find and repair the damage.
(4) In the conventional structure, a faulty section is detected by patrol after the occurrence of a fault with no relation to the presence or absence of a ground discharge or the like, to cope with the fault. In such case, a damaged portion of an optical fiber composite overhead ground wire may be unexpectedly detected. If an optical fiber composite overhead ground wire which has been struck by lightning does not develop any fault, any damage of the overhead ground wire caused by the lightning can be detected only by a routine inspection by patrollers. If no damaged portion is found through such routine inspection by the patrollers, the damage of the optical fiber composite overhead ground wire is completely overlooked. Therefore, it is desirable to provide an apparatus and a system which can detect a spot of an optical fiber composite overhead ground wire that has been struck by lightning, for finding a damaged portion of the optical fiber composite overhead ground wire for preventing the occurrence of a communication failure or the like even if the lightning does not cause any fault.
(5) In the system shown in FIG. 3A, it is necessary to install a plurality of detectors along the transmission line, since a transmission path is required for transmitting data obtained by these detectors. Such detectors must be installed after the transmission line is laid. Further, it is necessary to cut an optical fiber cable in portions provided with the detectors, in order to transmit the data from the respective detectors to other spots. Thus, the conventional system is complicated in structure and requires a troublesome operation for installing the system, while the system price is increased.